Photolithography is utilized in the fabrication of semiconductor devices to transfer a pattern onto a wafer. Based on various integrated circuit (IC) layouts, patterns are transferred from a photomask to a surface of the wafer. The photomasks, also called reticles, are made of quartz or glass with one or more metallic materials deposited on one side to prevent light penetration. As dimensions decrease and density in IC chips increases, resolution enhancement techniques, such as optical proximity correction (OPC), off-axis illumination (OAI), double dipole lithography (DDL) and phase-shift mask (PSM), are developed to improve depth of focus (DOF) and therefore to achieve a better pattern transfer onto the wafer.